This works, but it's the most counter-intuitive Reload button ever invented. It's in a different, seemingly unrelated app, in a settings screen, and on top of that it's a hidden gesture. What is Apple thinking?
– TobiaApr 8 '16 at 9:24

That is next level crazy/counter intuitive. After returning to the Reminders app I had to wait 5-10 seconds before it decided to sync in the background. This makes me think that syncing the calendar doesn't actually bring in the Reminders data, but somehow lets Reminders know that it needs to reach out to iCloud and sync itself. Anyway, it worked...
– J. VenatorApr 27 '17 at 23:00

I'm also experiencing slow Reminders syncing from my iPhone to my MacBook. I had my iCloud account in the Calendar app set to sync hourly because I don't use the calendar much. I wonder if that setting applies to Reminders too?
– Taylor EdmistonSep 24 '18 at 16:15

I think the reminders functionality started out in Calendar, possibly back when it was called iCal. Behind the scenes, they must still be tied in to each other.
– Paul D. WaiteJan 9 at 19:53

I found that adding a dummy item in a list on my iPhone initiates a sync, i.e. it fetches changes from other devices. Similarly, adding items in my iPhone updates my iMac and iPad instantly. The issue seem to be the iPhone not initiating sync without local changes on the iPhone. Note that on your iPhone, sync will not happen until after you press Done (or possibly pressing return is enough) or select another list.

Unfortunately, from what I can find, there isn’t any way to manually sync reminders between devices; it just happens. There are some things that you can do, however (assuming you are on iOS 8.3 and OS X Yosemite 10.10.3):

Make sure that both your iOS devices and your Mac are synced with the
same iCloud account. On your iOS account, go to Settings > iCloud and
make sure that Reminders is on. Then go to your Mac and go to System
Preferences, then click on iCloud and make sure that Reminders is on.

If Reminders is on, turn them off on both devices. I personally like
to wait about 5 minutes before turning them back on. Things should reset and start working again.

These are the only solutions that I have right now. If I think of more, I'll edit this answer and add them.

Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).